Display with dummy edge electrode

ABSTRACT

In an electroluminescent display device comprising layers of anode electrodes ( 102 ), cathode electrodes ( 104 ), and an electroluminescent layer ( 105 ), a dummy electrode ( 103 ) is arranged along the edge of te anode electrode layer. The dummy electrode is unconnected to the electrical control means ( 106,107 ) that control the voltage of the electrodes of the active area of the display. The unconnected state of the dummy electrode results in a situation where the dummy electrode attains a floating electric potential owing to the lateral electric field produced by neighboring electrodes. This leads to a reduction in the strength of the lateral electric field between the dummy electrode and the adjacent electrodes, thus alleviating the negative effects of electrochemical oxidation of the electrodes at the edge of the active area.

The present invention relates to an electroluminescent device and adisplay device comprising a first and a second layer of conductiveelectrodes and an active layer comprising electroluminescent material,said active layer being located between said first and second layers,said electrodes being arranged for connection to respective electricalcontrol means and thereby defining an active area of the display.

Electroluminescent displays according to prior art comprise two flatlayers of elongate and parallel conductive electrodes, i.e. an anodelayer and a cathode layer. The electrodes are usually arranged to form amatrix of picture elements (pixels) for use in a matrix display device.Commonly, the direction of elongation of the anodes is perpendicular tothe direction of elongation of the cathodes, and pixels are formed atthe crossings of an anode and a cathode. The electrode layers areproduced by techniques known in the art, such as sputtering,evaporation, and lithographic techniques.

An organic electroluminescent layer is located between the electrodelayers. Preferably, said organic electroluminescent layer comprises anelectroluminescent polymer material. The polymer layer is produced, forexample by known spin coating techniques or by an ink-jet printingmethod. Electrical control means drive the electrodes such thatpotential differences between anodes and cathodes result inelectroluminescence in the organic electroluminescent layer.

A specific prior art device is disclosed in the Japanese patentapplication having publication number 2000-012238. An organicelectroluminescent matrix element comprises a number of elongateparallel electrodes. The voltage of each electrode is controlled suchthat a desired electroluminescence is obtained in an electroluminescentlayer adjacent to the electrodes. A dummy electrode is located at theedge of the parallel electrodes. The voltage of the dummy electrode iscontrolled such that a reverse voltage, with respect to an adjacent edgeelectrode, is always present. Due to the reverse voltage, noelectroluminescence is obtained via the dummy electrode.

It has been observed that, the light output of the pixels at the edge ofthe display decreases over a time period which is significantly lessthan the lifetime of such a matrix display device.

An object of the invention is to overcome the drawbacks related to priorart devices as discussed above. This object is achieved in an inventivemanner by a device as claimed in the appended claims.

According to a first aspect, the invention relates to anelectroluminescent device comprising a first and a second layer ofconductive electrodes and an active layer comprising electroluminescentmaterial, said active layer being located between said first and secondlayers, said electrodes being arranged for connection to respectiveelectrical control means and thereby defining an active area of thedevice. The first electrode layer further comprises at least one dummyelectrode, said dummy electrode being arranged to be disconnected fromsaid electrical control means and located at least partly along an outeredge of said active area.

An effect of the invention is that it counteracts the negative effectsthat produce the problem of a reduced lifetime of a display as discussedabove. The negative effects are due to an increased potential dropacross the electrode located at the edge of the active area. Thepotential drop leads to a decrease in potential difference across theelectroluminescent layer, resulting in a decreased light output from theelectroluminescent material between the cathode and anode layers.

The potential drop across the electrode is probably due to an increasedinternal resistance caused by electrochemical oxidation of the materialin the electrode. It can be noted that the rate of electrochemicaloxidation of the edge electrode is dependent on the strength of thelateral electric field (i.e. perpendicular to the direction ofelongation of the electrode) present at the edge.

Hence, in order to reduce the strength of such a lateral electric fieldat the edge electrode, a dummy electrode is arranged along the edge ofthe active area of the display. The dummy electrode is disconnected fromthe electrical control means that control the voltage of the electrodesof the active area of the display. The unconnected state of the dummyelectrode results in a situation where the dummy electrode attains avarying electric potential i.e. a floating potential, owing to thelateral electric field produced by neighboring electrodes. This leads toa reduction in the strength of the lateral electric field between thedummy electrode and the adjacent electrodes of the active area, thusalleviating the negative effects of electrochemical oxidation of theelectrodes at the edge of the active area.

An advantage of the present invention is that it increases the lifetimeof an electroluminescent display device. That is, the deterioration rateof the anode electrodes at the edge of the active area becomescomparable to the deterioration rate of the anode electrodes within theactive area, hence leading to an even light output across the activearea.

In an embodiment of the invention, the dummy electrode is totallydisconnected from any conductor, thereby providing an advantage of beingvery simple to manufacture.

Although the invention as stated above relates to a device of anygeometrical shape, a preferred embodiment is that of a matrixconfiguration, while at the same time the dummy electrode is of the sameshape and geometrical extent as the electrodes of the active area.

Such an embodiment also has the advantage that is more simple tomanufacture. Furthermore, it allows to provide dummy electrodes withreduced size for reasons of space reduction. A smaller shaped dummyelectrode has the advantage of a smaller passive area (and thus smallerdevice) than the dummy electrode of the same shape.

Moreover, by suitably selecting a polymer material, such aspolyethylenedioxythiophene, as a constituent part of the material, ofthe electrodes, an efficient electroluminescence can be obtained.

For reason of clarity, the EL device comprises one or more functionallayers. Examples of such functional layers are an electroluminescent,charge transport and charge injecting layers. In order to fully exploitthe benefits of the invention, the one or more functional layers arepreferably provided using a wet deposition method.

The one or, if there is more than one, at least one of the functionallayers is an electroluminescent layer. The EL layer is made of asubstantially, preferably organic, electroluminescent material. In thecontext of the invention, the type of EL material used is not criticaland any EL material known in the art can be used. Preferably, howeverobtainable from a fluid which can be deposited using a wet depositionmethod. Suitable organic EL materials include organic photo- orelectroluminescent, fluorescent and phosphorescent compounds of low orhigh molecular weight. Suitable low molecular weight compounds are wellknown in the art and include tris-8-aluminium quinolinol complex andcoumarins. Such compounds can be applied using vacuum-deposition method.Alternatively, the low molecular weight compounds can be embedded in apolymer matrix or chemically bonded to polymers, for example byinclusion in the main chain or as side-chains, an example beingpolyvinylcarbazole.

Preferred high molecular weight materials contain EL polymers having asubstantially conjugated backbone (main chain), such as polythiophenes,polyphenylenes, polythiophenevinylenes, or, more preferably,poly-p-phenylenevinylenes. Particularly preferred are (blue-emitting)poly(alkyl)fluorenes and poly-p-phenylenevinylenes which emit red,yellow or green light and are 2-, or 2,5- substitutedpoly-p-phenylenevinylenes, in particular those havingsolubility-improving side groups at the 2- and/or 2,5 position such asC₁-C₂₀, preferably C₄-C₁₀, alkyl or alkoxy groups. Preferred side groupsare methyl, methoxy, 3,7-dimethyloctyloxy, and 2-methylpropoxy. Moreparticularly preferred are polymers including a2-aryl-1,4-phenylenevinylene repeating unit, the aryl group beingoptionally substituted with alkyl and/or alkoxy groups of the typeabove, in particular methyl, methoxy, 3,7-dimethyloctyloxy, or, betterstill, 2-methylpropoxy. The organic material may contain one or more ofsuch compounds. Such EL polymers can suitably be applied by wetdeposition techniques.

In the context of the invention, the term organic includes polymericwhereas the term polymer and affixes derived therefrom, includeshomopolymer, copolymer, terpolymer and higher homologues as well asoligomer.

Optionally, the organic EL material contains further substances, organicor inorganic in nature, which may be homogeneously distributed on amolecular scale or present in the form of a particle distribution. Inparticular, compounds improving the charge-injecting and/orcharge-transport capability of electrons and/or holes, compounds toimprove and/or modify the intensity or color of the light emitted,stabilizers, and the like may be present.

The organic EL layer preferably has an average thickness of 50 nm to 200nm, in particular, 60 run to 150 nm or, preferably, 70 nm to 100 nm.

Optionally, the EL device comprises further, preferably organic,functional layers disposed between the electrodes. Such further layersmay be hole-injecting and/or transport (HTL) layers andelectron-injecting and transport (ETL) layers. Examples of EL devicescomprising more than one functional layer are a laminate of anode/HTLlayer/EL layer/cathode, anode/EL layer/ETL layer/cathode, or anode/HTLlayer/EL layer/ETL layer/cathode.

Suitable materials for the hole-injecting and/or hole-transport layers(HTL) include aromatic tertiary amines, in particular diamines or higherhomologues, polyvinylcarbazole, quinacridone, porphyrins,phthalocyanines, poly-aniline and poly-3,4-thylenedioxythiophene.

Suitable materials for the electron-injecting and/or electron-transportlayers ETL) are oxadiazole-based compounds and aluminiumquinolinecompounds.

If ITO is used as the anode, the EL device preferably comprises a 50 to300 nm thick layer of the hole-injecting/-transport layer materialpoly-3,4 thylenedioxythiophene or a 50 to 200 nm thick layer ofpolyaniline.

Generally, the EL device comprises a substrate. Preferably, thesubstrate is transparent with respect to the light to be emitted.Suitable substrate materials include transparent synthetic resin whichmay or may not be flexible, quartz, ceramics and glass. The substrateprovides the supporting surface for the relief pattern.

Although in its broadest sense, the invention is applicable to ELdevices having a single electroluminescent area, the invention isparticularly beneficial for an electroluminescent device comprising aplurality of light emitting areas. The accompanying sections have thefunction of preventing functional layer material from being deposited inthe wrong light emitting are and/or in areas where no light emission isto occur.

For display purposes, the separate light emitting areas are referred toas EL elements or pixels and are generally independently addressable.Each EL element has an area capable of light emission. The lightemissive area is part of the EL layer. A light emissive area is formedby the area of overlap of a first electrode, a second electrode, and anorganic EL layer. The EL device may be a segmented or matrix displaydevice of the passive or active type.

The first electrode layer may be electron-injecting and the secondelectrode layer hole-injecting. Alternatively, the first electrode layeris hole injecting and the second electrode layer is electron injecting.

An electron-injecting electrode is suitably made of a metal(alloy)having a low work function, such as Yb, Ca, Mg:Ag Li:Al, Ba or is alaminate of different layers such as Ba/Al or Ba/Ag electrode.

A hole-injecting electrode is suitably made of a metal (alloy) having ahigh work function such as Au, Pt, Ag. Preferably, a more transparenthole-injecting electrode material, such as an indiumtinoxide (ITO), isused. Conductive polymers such as a polyaniline (PANI) and apoly-3,4-ethylenedioxythiophene (PEDOT) are also suitable transparenthole-injecting electrode materials. Preferably, a PANI layer has athickness of 50 to 200 nm, and a PEDOT layer 100 to 300 nm. If an ITOhole-injecting electrode is used, the first electrode is preferably thehole-injecting electrode.

According to a second aspect of the present invention, a display deviceis provided. The display device comprises a substrate layer on which afirst and a second layer of conductive electrodes and an active layercomprising electroluminescent material are arranged, said active layerbeing located between said first and second layers, said electrodesbeing connected to electrical control means and thereby defining anactive area of the display. The first electrode layer further comprisesat least one dummy electrode, said dummy electrode being disconnectedfrom said electrical control means and located at least partly along anouter edge of said active area.

The technical effects and advantages of such a device have beendiscussed with reference to the electroluminescent device above.

The invention will now be described in terms of preferred embodiments,and reference will be made to the drawings, where:

FIG. 1 is a diagrammatic a perspective view of an electroluminescentdevice according to the invention, and

FIG. 2 is a diagram of a display device according to the invention.

FIG. 1 illustrates diagrammatically an embodiment of anelectroluminescent device according to the invention. The devicecomprises a substrate 101, which may be of any material known in theart, including glass materials, and may be appropriately chosen by theskilled person. On the substrate 101 are located a number of parallelanode electrodes 102, i.e. an anode electrode layer. Although only a feware illustrated in FIG. 1, the skilled person will realize that thenumber of anode electrodes 102 may be much greater and counted inhundreds, if not thousands. Moreover, the anode electrodes 102 areproduced by techniques known to the skilled person, such as sputtering,evaporation, and lithographical techniques and may consist at leastpartly of a polymer material, for example polyethylenedioxythiophene.

The anode electrodes 102 are all connected to an anode control unit 106which provides electric voltages to the anode electrodes 102, as theskilled person realizes.

On the anode electrodes 102 there is an electroluminescent layer 105.The electroluminescent layer 105 is manufactured in a manner known tothe skilled person and is, for example, of an appropriately chosenpolymer material.

On the electroluminescent layer 105 are located a number of parallelcathode electrodes 104. As for the anode electrodes 102 discussed above,only a few are illustrated in FIG. 1, and the skilled person willrealize that the number may be much greater. Moreover, the cathodeelectrodes 104 are also produced by techniques known to the skilledperson, such as sputtering, evaporation, and lithographical techniques.

The cathode electrodes 104 are all connected to a cathode control unit107 which provides electric voltages to the cathode electrodes 104, asthe skilled person realizes, and in combination with the anode controlunit 106 drives the electrodes such that potential differences betweenanode electrodes and cathode electrodes result in electroluminescence inthe electroluminescent layer 105.

The layer comprising anode electrodes 102 further comprises two dummyanode electrodes 103 which are not connected to the anode electrodecontrol unit 106. These dummy anode electrodes 103 are located adjacentto the anode electrodes 102.

The effect of these unconnected dummy anode electrodes 103 is that theirpresence counteracts the negative effects that produce the problem ofreduced lifetime of a display as discussed above. The negative effectsare due to a strong lateral electric field at the edge anode electrodes102 located adjacent to the dummy anode electrodes 103. Without thepresence of the dummy anode electrodes 103, the strong lateral electricfield would lead to an increased deterioration rate and hence a decreasein potential difference across the electroluminescent layer 105,resulting in a decreased light output, as was discussed above.

In FIG. 2, a second embodiment of the present invention isdiagrammatically shown. A display unit 201 is shown suitable for use ina computer, mobile communication terminal, or indeed in any electronicequipment that requires a display screen. For simplicity, the equipmentto which the display unit 201 is connected is illustrated as a genericuser unit 208 in FIG. 2.

The display unit comprises an electroluminescent device as describedabove with reference to FIG. 1. This is a device on a substrate 202, inwhich an electroluminescent layer 206 is located between layers of anodeelectrodes 203 and cathode electrodes 205. A control unit 207 controls,in cooperation with the user equipment 208, the voltages applied to theanode electrodes 203 and the cathode electrodes 205 in such a mannerthat an active area 209 is obtained.

The layer comprising anode electrodes 203 further comprises two dummyanode electrodes 204 that are not connected to the control unit 207.These dummy anode electrodes 204 are located adjacent to the anodeelectrodes 203 at the edges of the active area 209, resulting in theeffect that the lifetime of the anode electrodes 203 at the edge of theactive area 209 is increased.

Finally, in summary, an electroluminescent display device has beendescribed, comprising layers of anode electrodes, cathode electrodes,and an electroluminescent layer, a dummy electrode being arranged alongthe edge of the anode electrode layer. The dummy electrode isdisconnected from to the electric control means that control the voltageof the electrodes of the active area of the display. The unconnectedstate of the dummy electrode results in a situation where the dummyelectrode attains a floating electric potential owing to the lateralelectric field produced by neighboring electrodes. This leads to areduction in the strength of the lateral electric field between thedummy electrode and the adjacent electrodes, thus alleviating thenegative effects of electrochemical oxidation of the electrodes at theedge of the active area. Herewith it is achieved that during operationof the device the light intensity at the edge of the device ismaintained at the original level. The problem of pixel shrinkage istherewith at least substantially reduced. Also, an extension of thelifetime is obtained.

1. Electroluminescent device comprising a first and a second layer ofconductive electrodes (102,104,203,205) and an active layer comprisingelectroluminescent material (105,206), said active layer being locatedbetween said first and second layers, said electrodes being arranged forconnection to respective electrical control means (106,107,207) andthereby defining an active area (209) of the device, characterized inthat the first electrode layer further comprises at least one dummyelectrode (103,204), said dummy electrode being arranged to bedisconnected from said electrical control means and located at leastpartly along an outer edge of said active area.
 2. An electroluminescentdevice as claimed in claim 1, said at least one dummy electrode beingarranged so as to be unconnected to any conductor.
 3. Anelectroluminescent device as claimed in claim 1, said electrodes of thefirst layer being elongate and substantially parallel, said electrodesof the second layer being elongate and substantially parallel, saidelectrodes of the first layer being substantially perpendicular to theelectrodes of the second layer, thereby defining a matrix display, andsaid at least one dummy electrode being elongate and substantiallyparallel to the electrodes of the first layer.
 4. An electroluminescentdevice as claimed in claim 3, said dummy electrode being substantiallyof equal spatial extent as the electrodes of the first layer.
 5. Anelectroluminescent device as claimed in claim 1, said electrodes of thefirst layer comprising polyethylenedioxythiophene.
 6. A display device(201) comprising a substrate layer (202) on which a first and a secondlayer of conductive electrodes (203,205) and an active layer comprisingelectroluminescent material (206) are arranged, said active layer beinglocated between said first and second layers, said electrodes beingconnected to electrical control means (207) and thereby defining anactive area (209) of the display, characterized in that the firstelectrode layer further comprises at least one dummy electrode (204),said dummy electrode being disconnected from said electrical controlmeans and located at least partly along an outer edge of said activearea.